Short interest in Comcast Corp has jumped in recent weeks as some investors bet that the rise in the cable company's shares following its NBC Universal deal has reached a peak.

Short positions in Comcast rose 24 percent or 10.6 million shares in the two weeks to December 15, making the company the third-biggest increase in short interest on Nasdaq.

This comes after the shares gained as much as 18 percent after Comcast announced its deal on December 3 to take a 51 percent stake in NBC Universal. Comcast would spend around $6.5 billion in cash and contribute cable networks worth $7.25 billion.

Analysts said investors are now waiting to see how the regulatory review unfolds, a process that could take from nine months to 18 months.

Some shorts may be assuming deal enthusiasm is possibly going to wane because of the time it's going to take till the deal gets done, said David Joyce, an analyst at Miller Tabak.

Short traders typically borrow shares with the view to sell them at a high and hope to make a profit by buying them back at a lower price and then return the shares to the original owner.

Short traders are betting that Comcast's shares are now overvalued and hope to profit if the price drops.

When a company makes a sizable acquisition, its shares usually fall as investors worry that it may be piling on too much debt or making a bad strategic move said Tuna Amobi, an analyst at Standard & Poor's.

But Wall Street has been generally positive on Comcast following the NBC Universal deal announcement, which dispelled fears that the cable company would spend a lot of cash to own a media conglomerate. In theory, Comcast can avoid a hefty cash bill to buy the remaining 49 percent from General Electric Co if NBC Universal meets expected operating targets.

The shares have outperformed expectations since the merger date, said Amobi.

Collins Stewart analyst Thomas Eagan estimates Comcast currently trades at around 5.35 times forecast 2010 operating cash flow, similar to Time Warner Cable which trades at 5.4 times 2010 cash flow.

But if NBC Universal is included in Comcast's numbers, the company is trading at a higher multiple of around 6.4 times, by Collins Stewart's estimates.

With the recent run-up in Comcast stock, it could be there's concern that it went up by too much, Eagan said.

People who are bearish on the stock may think you're not going to see this kind of appreciation going forward, he added.

Comcast shares have lost some of the gains made in the week following the announcement of the deal. On Tuesday its shares were trading at around $17.16, up 14.5 percent from the morning of Dec 3.

(Reporting by Yinka Adegoke, editing by Tiffany Wu and Tim Dobbyn)

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